This invention relates to a water-based maskant composition which is excellent in coatability, excellent in film strength and mask-dimension precision during etching, excellent in mask-removing workability, and also free from problems of environmental hygiene because of no generation of detrimental gases, and to a chemical milling method using the water-based maskant composition.
Metal chemical milling is well known in which strong acids or bases are used to etch away the unneeded portions of a metal article. In particular, in aircraft industries, reduction of the weight of aircraft parts has been required and the metal chemical milling has been used therefor.
The metal chemical milling is usually conducted by the following steps:
(1) A mask resistant to an etching bath to be used is formed on the base metal by a means such as immersion or the like. PA1 (2) The mask thus formed is scribed using an appropriate template, to allow the desired portions of the mask to be peeled away to selectively expose the metal portions to be etched. PA1 (3) Subsequently, the exposed metal portions are etched away with an etchant such as an alkali bath for aluminum parts or an acid bath for titanium parts. PA1 (4) The mask is removed. PA1 (a) The circumferential portions of the mask tend to be swollen with an alkali bath which is an etchant, and it is impossible to conduct chemical milling with high dimension precision. PA1 (b) PA1 (c) The peelability of mask is poor, and hence, the mask-removing workability is inferior.
Heretofore, however, the maskant composition used in the above metal chemical milling method has been a rubber solution obtained by dissolving a rubber in an organic solvent, for example, toluene/xylene or perchloroethylene (see U.S. Pat. Nos. 3,380,863, 3,649,584 and the like).
Accordingly, in the course of applying and drying the maskant composition, the organic solvent is evaporated and, as a result, serious problems are caused in respect of environmental hygiene such as odor, air pollution and the like.
Recently, research has thus been made on use of a water-based maskant composition which is free from organic solvent and comprises an aqueous rubber latex as a main component in place of the above rubber solution.
Examples in which such aqueous rubber latexes are used are those of U.S. Pat. Nos. 4,373,050, 4,806,390, 4,946,711, EP 517,983 (corresponding to U.S. Ser. No. 713,165) and the like.
In U.S. Pat. No. 4,373,050, there is used a water-based acidic coating composition in which particles of a polymer such as styrene-butadiene copolymer, acrylonitrile-butadiene copolymer, polyethylene, polyacrylic acid or the like are dispersed; in U.S. Pat. Nos. 4,806,390 and 4,946,711, a neoprene latex is used; and in EP 517,983, a mixture of styrene-butadiene latex and neoprene latex is used.
However, when the above water-based maskant compositions are used, it has been impossible to sufficiently solve the following problems:
Since the coatability of the water-based maskant composition is poor, the coating workability is inferior.